The present invention generally relates to the detection of alpha radiation and, more specifically, to the detection of radon or other sources of alpha radiation through the counting of alpha events occurring inside a decay cavity.
It has been found that, on average, radon constitutes more than half of the normal background radioactivity in the environment. Gaseous .sup.222 radon, produced by natural radium in the ground, is the isotope primarily responsible for such radioactivity. When .sup.222 radon enters a house, either through small fissures, or in well water, it can remain in the air if not ventilated. In such a case, the .sup.222 radon decays to non-gaseous radioactive daughter products that can be absorbed in the lungs and can lead, over a long period of exposure, to lung cancer. The U.S. Environmental Protection Agency has estimated that between 7,000 and 30,000 of the 130,000 lung cancer deaths annually may be attributed to indoor radon. The Environmental Protection Agency recommends remedial action if radon levels in homes exceed 4 trillionths of a curie, or 4 picocuries of radon per liter (pCi/L) of air. Although initial discoveries pointed to particular areas susceptible to radon pollution, it is now known that high concentrations exist in many parts of the United States, as well as in a number of European countries.
The recent public awareness concerning the existence of radon gas has brought about a demand for sensitive and accurate detection instruments. At the present, there are numerous commercial offerings of devices intended to detect radon, the devices having varying degrees of effectiveness. All of these devices suffer from one or more of the following problems: (1) lack of sensitivity, (2) slow response time, and (3) high cost.
The present invention provides a radon detection instrument which solves these problems. It is based on technology which is contained in several U.S. Patents which disclose various devices for the long range detection of alpha particles. The first is U.S. Pat. No. 5,184,019, issued Feb. 2, 1993, for a Long Range Alpha Particle Detector. The second is U.S. Pat. No. 5,194,737, issued Mar. 16, 1993, for Single and Double Grid Long Range Alpha Detectors. The third is U.S. Pat. No. 5,187,370, issued Feb. 16, 1993, for Alternating Current Long Range Alpha Particle Detectors. The fourth is U.S. Patent No. 5,281,824, issued Jan. 25, 1994, for Radon Detection. The fifth is U.S. Pat. No. 5,311,025, issued May 10, 1994, for Fan-less Long Range Alpha Detector. The principle underlying each of these patents is that alpha particles, although themselves of very short range in air, ionize various of the molecular species in air. These ions, referred to herein as "air ions," have a sufficiently long lifetime that they may be transported by mass flow of the surrounding air, and detected at distances much greater than the penetration distances of the original alpha particles by the apparatus disclosed in the above-referenced patents. The present invention modifies this apparatus to provide for reliable detection of radon through the electrostatic detection of individual pulses created by low level radon decay.
Although the previous long range alpha detectors are extremely efficient in detecting ions created by radiation, they are not as sensitive as possible because they are primarily direct current (dc) systems. This is because of the dc errors created within the detector itself. With the previous long range alpha detectors, the large radiation source strength created an output with a large number of overlapping pulses which, together with the instrumental noise and cosmic rays, smeared into a dc level that indicated the presence of alpha radiation, but from which one was unable to count individual alpha events.
In the present invention, which can detect relatively small alpha source strengths, ion pulses from individual alpha decays can be observed. This makes the invention an excellent detector for radon, or any other source of low strength alpha radiation. It is capable of rendering real time measurements, as opposed to the delays involved with conventional radon detectors, which must be forwarded to a laboratory for analysis after being in place for an extended period of time.
The energy lost by an alpha particle in ionizing an air molecule is approximately 35 eV, which indicates that a typical 5.5 MeV alpha decay from .sup.222 Rn will produce approximately 157,000 air ions. The ionized electron will quickly migrate to another air molecule, creating an ion pair consisting of two large, charged molecules. These ion pairs can be transported to an electrode where the current produced by these ion can be measured. If the alpha decay rate is sufficiently low to preclude pileup, current pulses produced by individual alpha decays can be measured. Since neither a cosmic ray nor instrumental noise can create the quantity of air ions necessary to produce a pulse, they will have no effect on the operation of the present invention.
It is therefore an object of this invention to present apparatus for counting individual alpha decay events.
It is another object of the present invention to present apparatus for the real time measurement of the presence of radon gas, or other low strength alpha radiation.
It is a further object of the present invention to provide a radon detector which is capable of high precision radon measurement over a wide range of concentrations.
It is a still further object of the present invention to provide a radon detector which has a short response time.
It is a feature of the present invention that it can be produced at low cost.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.